This invention relates to an FM radio receiver containing a radio frequency tuning circuit for selecting a signal from an antenna and a local oscillator tuning circuit tunable to an oscillating signal for converting the frequency of a receiving signal to an intermediate frequency.
In such a kind of super-heterodyne FM radio receiver, a desired broadcast signal received at an antenna is converted to a frequency called an intermediate frequency (IF), which is then detected and demodulated. For FM reception, 10.7 MHz is normally used as this intermediate frequency. A local oscillator circuit is oscillated at a frequency always apart from the frequency of a desired signal (that is, receiving frequency) by 10.7 MHz. A 10.7 MHz IF signal is generated as a beat component as a result of mixing the oscillating signal of this local oscillator circuit and the received signal so as to convert the received signal to the intermediate frequency. At this conversion, if the local oscillator circuit is oscillated at a frequency higher by 10.7 MHz than the receiving frequency, it is called the upper heterodyne method. On the other hand, if the local oscillator circuit is oscillated at a frequency lower by 10.7 MHz than the receiving frequency, it is called the lower heterodyne method.
Whether an FM radio receiver is designed in the upper heterodyne method or lower heterodyne method is determined depending on the frequency allocation for FM radio broadcasting in each country and the presence of interference signals at image frequencies. In a super-heterodyne FM radio receiver, if an interference signal is located at the image frequency which is 10.7 MHz apart from a local oscillating frequency in the opposite direction to a receiving frequency, (i.e., 21.4 MHz apart from the receiving frequency,) image signals are mixed into the intermediate frequency signal resulting in interference. In Japan, the frequency allocation for FM radio broadcasting is 76-90 MHz, and channel 2 of TV broadcasting is allocated at frequencies 21.4 MHz higher than the FM band. In the USA, the frequency allocation for FM radio broadcasting is 88-108 MHz, and channels 4 to 6 of TV broadcasting are allocated at frequencies 21.4 MHz lower than the FM band. Accordingly, to avoid image frequency interference by strong TV broadcasting signals with a high electric field strength, the lower heterodyne method is used in Japan and the upper heterodyne method is used in the US (and also in Europe).
A conventional FM radio receiver will now be described referring to a car radio receiver with a front-end circuit as shown in FIG. 6.
The front-end circuit of the conventional FM radio receiver comprises a radio frequency tuning circuit containing an antenna tuning circuit 61 and an RF tuning circuit 63, an RF amplifier circuit 62, a local oscillator circuit containing a local oscillator tuning circuit 64, and a mixer circuit 65. The antenna tuning circuit 61 comprises a tuning coil L9, a pair of variable capacitance diodes D13, D14 which cathodes are connected to each other, and a capacitor C11. The RF tuning circuit 63 comprises a tuning coil L10, a pair of variable capacitance diodes D15, D16 which cathodes are connected to each other, and a capacitor C12. The local oscillator tuning circuit 64 comprises an oscillator coil L11, a pair of variable capacitance diodes D17, D18 which cathodes are connected to each other, and a capacitor C13. The variable capacitance diodes D13, D14, D15, D16, D17, D18 have the same voltage versus capacitance characteristic. A common tuning voltage from a source 66 is applied to each pair of the variable capacitance diodes of the antenna tuning circuit 61, RF tuning circuit 63, and local oscillator tuning circuit 64. Generally, this tuning voltage is set in a range of from 1 to 8 volts which can be supplied stably from a vehicle battery.
For an FM radio receiver which is intended for use in Japan, a value of each element is determined so that, within a tuning voltage range (e.g., 1-8 V) applied to the pair of the variable capacitance diodes of each tuning circuit, the antenna tuning circuit 61 and RF tuning circuit 63 tune to FM radio broadcasting signals of 76-90 MHz and the local oscillator tuning circuit 64 tunes to oscillating signals (i.e., 65.3-79.3 MHz) which are lower by 10.7 MHz than the tuning frequencies of the antenna tuning circuit 61 and RF tuning circuit 63.
For an FM radio receiver which is intended for use in the US, a value of each element is determined so that, within a tuning voltage range (e.g., 1-8 V) applied to the pair of the variable capacitance diodes of each tuning circuit, the antenna tuning circuit 61 and RF tuning circuit 63 tune to FM radio broadcasting signals of 88-108 MHz and the local oscillator tuning circuit 64 tunes to oscillating signals (i.e., 98.7-118.7 MHz) which are higher by 10.7 MHz than the tuning frequencies of the antenna tuning circuit 61 and RF tuning circuit 63.
For the FM radio receivers described above, the receiving bands are different between receivers for Japan and the US, and, moreover, the oscillating frequencies of the local oscillator circuits are entirely different because of a difference between upper heterodyne and lower heterodyne methods. Consequently, as shown in Table 1, the values of the coil elements and capacitance elements in the radio frequency tuning circuit and the local oscillator tuning circuit are significantly different for manufacturing the separate receivers. It is noted that, for both cases in Table 1, variable capacitance diodes used in each tuning circuit are those whose capacitance values change from 12 to 22.5 pF when the tuning voltage is changed from 1 to 8 V.
TABLE 1 radio frequency tuning local oscillator tuning circuit circuit Capacitance Oscillator Capacitance tuning coil element coil element L9, L10 C11, C12 L11 C13 for Japan 120.476 nH 14.03 pF 182.127 nH 10.117 pF for US 104.258 nH 8.77 pF 76.42 nH 11.525 pF
Therefore, conventional FM radio receivers need to be designed and manufactured separately for Japan and the US. Consequently, more design work is involved and the handling of components used in manufacturing the FM radio receivers becomes complicated since more components are required, thereby spoiling productivity.